The present invention relates to a novel method for preparing nitrogen-containing compounds (e.g., isothioureas, guanidines, amidines, amides) using amines.
Nitrogen-containing compounds including isothioureas, guanidines, amidines and amides are extremely important in the fields of pharmaceutical or agricultural synthesis. Typical techniques used for preparing these compounds from amines in a single step are shown below.
No technique is known to allow the direct synthesis of isothioureas from amines and thiocyanates.
A technique for synthesizing guanidines from amines and cyanamides is known from J. Med. Chem., 1996, 39, 4017, which reports reaction conditions involving heating the reaction mixture to 180xc2x0 C. in concentrated hydrochloric acid.
A technique for synthesizing amidines from amines and nitrites is known from Chem. Pharm. Bull., 1997, 45, 987, which reports reaction conditions involving heating the reaction mixture to 150xc2x0 C. in the presence of aluminum chloride.
A technique for synthesizing amides from amines and esters is known from Tetrahedron Lett., 1996, 37, 2757, which reports the use of trimethylaluminum for the synthesis. Also, another synthesis technique using a silylating agent is known from Tetrahedron Lett., 1991, 32, 3407, which reports the use of trimethylsilyl chloride for the synthesis. However, in these techniques, the silylating agent is used for the protection of other functional groups and does not appear to enhance the synthesis reaction because the esters to be amidated are reactive enough to easily react with the amines in the absence of the silylating agent. It is therefore impossible to predict from these techniques that the silylating agent enhances the amidation reaction of less reactive esters. Still another technique using tin bishexamethylsilylamide is known from J. Org. Chem., 1992, 57, 6101. However, this technique is based on a concept distinct from that of the synthesis reaction under consideration because the active species is tin.
These conventional techniques require the use of highly reactive metal compounds and/or extreme reaction conditions for the direct synthesis of nitrogen-containing compounds (e.g., isothioureas, guanidines, amidines, amides) from amines. Such conventional techniques are therefore unable to have a wide range of applications and have been unsuitable for large-scale synthesis in terms of running costs, energy consumption and environmental impact. In view of the foregoing, there has been a demand to develop a preparation method that is available for a wider range of applications and that allows the efficient preparation of nitrogen-containing compounds including isothioureas, guanidines, amidines and amides under mild reaction conditions.
As a result of our research efforts directed to overcoming the problems stated above, we found that the use of a silylating agent for catalyzing the reactions between NH group-containing compounds (e.g., amines) and thiocyanates, cyanamides, nitrites or esters enabled the direct and efficient synthesis of nitrogen-containing compounds of interest under mild reaction conditions. We also found that this synthesis technique using a silylating agent had a wide range of applications and was suitable for large-scale synthesis. The present invention has been accomplished on the basis of these findings.
In short, the present invention provides a method for preparing isothioureas, guanidines, amidines or amides, which comprises reacting a NH group-containing compound with a compound selected from the group consisting of thiocyanates, cyanamides, nitrites and esters (excluding highly reactive esters) in the presence of a silylating agent.
Also, the present invention provides a method for preparing isothioureas and/or tautomers thereof, in which a NH group-containing compound of general formula (I-A):
R1aR2aNHxe2x80x83xe2x80x83(I-A)
wherein
R1a and R2a, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1aR2aN represents an optionally substituted monovalent cyclic hydrocarbon residue,
is reacted with a thiocyanate compound of general formula (II-A):
R3aSCNxe2x80x83xe2x80x83(II-A)
wherein
R3a, which is the same or different, represents a hydrogen atom or an optionally substituted monovalent hydrocarbon residue,
in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give an isothiourea compound of general formula (III-A) and/or a tautomer thereof: 
wherein
R1a, R2a and R3a are as defined above.
Further, the present invention provides a method for preparing guanidines and/or tautomers thereof, in which a NH group-containing compound of general formula (I-B):
R1bR2bNHxe2x80x83xe2x80x83(I-B)
wherein
R1b and R2b, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1bR2bN represents an optionally substituted monovalent cyclic hydrocarbon residue,
is reacted with a cyanamide compound of general formula (II-B):
R3bR4bNCNxe2x80x83xe2x80x83(II-B)
wherein
R3b and R4b, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R3bR4bN represents an optionally substituted monovalent cyclic hydrocarbon residue,
in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give a guanidine compound of general formula (III-B) and/or a tautomer thereof: 
wherein
R1b, R2b, R3b and R4b are as defined above.
In addition, the present invention provides a method for preparing amidines and/or tautomers thereof, in which a NH group-containing compound of general formula (I-C):
R1cR2cNHxe2x80x83xe2x80x83(I-C)
wherein
R1c and R2c, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1cR2cN represents an optionally substituted monovalent cyclic hydrocarbon residue,
is reacted with a nitrile compound of general formula (II-C):
R3cCNxe2x80x83xe2x80x83(II-C)
wherein
R3c represents a hydrogen atom or an optionally substituted monovalent hydrocarbon residue,
in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give an amidine compound of general formula (III-C) and/or a tautomer thereof: 
wherein
R1c, R2c and R3c are as defined above.
Furthermore, the present invention provides a method for preparing amides, in which a NH group-containing compound of general formula (I-D):
R1dR2dNHxe2x80x83xe2x80x83(I-D)
wherein
R1d and R2d, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1dR2dN represents an optionally substituted monovalent cyclic hydrocarbon residue,
is reacted with an ester compound (excluding highly reactive esters) of general formula (II-D):
R3dCO2R4dxe2x80x83xe2x80x83(II-D)
wherein
R3d represents a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, R4d represents a hydrogen atom, an optionally substituted monovalent hydrocarbon residue or a substituted silyl group, or R3dCO2R4d represents an optionally substituted cyclic hydrocarbon,
in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give an amide compound of general formula (III-D): 
wherein
R1d, R2d and R3d are as defined above.
The method of the present invention can be applied where NH group-containing compounds are reacted with thiocyanates, cyanamides, nitriles or esters to synthesize nitrogen-containing addition or substitution products thereof.
As used herein, a NH group-containing compound literally refers to any compound containing NH, including a linear or cyclic primary amine, a linear or cyclic secondary amine and a linear or cyclic imide.
As used herein, thiocyanates, cyanamides, nitrites and esters are intended to include, for example, compounds represented by general formulae (II-A), (II-B), (II-C) and (II-D), respectively.
As used herein, isothioureas, guanidines, amidines and amides are intended to include, for example, compounds represented by general formulae (III-A), (III-B), (III-C) and (III-D), respectively. According to the present invention, these isothioureas, guanidines, amidines and amides are obtained through the addition or substitution reactions of NH group-containing compounds with thiocyanates, cyanamides, nitriles and esters, respectively. More specifically, isothioureas are derived from thiocyanates, guanidines are derived from cyanamides, amidines are derived from nitriles, and amides are derived from esters.
According to the present invention, the nitrogen-containing compounds including isothioureas, guanidines, amidines and amides can be synthesized as follows.
Preparation of Isothioureas
A compound of general formula (I-A):
R1aR2aNHxe2x80x83xe2x80x83(I-A)
wherein
R1a and R2a, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1aR2aN represents an optionally substituted monovalent cyclic hydrocarbon residue,
can be reacted with a compound of general formula (II-A):
R3aSCNxe2x80x83xe2x80x83(II-A)
wherein
R3a represents a hydrogen atom or an optionally substituted monovalent hydrocarbon residue,
in an inert solvent or without using a solvent in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give an isothiourea compound of general formula (III-A) and/or a tautomer thereof: 
wherein
R1a, R2a and R3a are as defined above.
The tautomer as used herein refers to a compound of general formula (III-Axe2x80x2): 
which may be generated when R2a in general formula (III-A) is a hydrogen atom. Exactly the same can be said for the case where R1a in general formula (III-A) is a hydrogen atom.
Preparation of Guanidines
A compound of general formula (I-B):
R1bR2bNHxe2x80x83xe2x80x83(I-B)
wherein
R1b and R2b, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1bR2bN represents an optionally substituted monovalent cyclic hydrocarbon residue,
can be reacted with a compound of general formula (II-B):
R3bR4bNCNxe2x80x83xe2x80x83(II-B)
wherein
R3b and R4b, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R3bR4bN represents an optionally substituted monovalent cyclic hydrocarbon residue,
in an inert solvent or without using a solvent in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give a guanidine compound of general formula (III-B) and/or a tautomer thereof: 
wherein
R1b, R2b, R3b and R4b are as defined above.
The tautomer as used herein refers to a compound of general formula (III-Bxe2x80x2) or (III-Bxe2x80x3): 
which may be generated when R2b or R4b in general formula (III-B) is a hydrogen atom. Exactly the same can be said for the case where R1b or R3b in general formula (III-B) is a hydrogen atom.
Preparation of Amidines
A compound of general formula (I-C):
R1cR2cNHxe2x80x83xe2x80x83(I-C)
wherein
R1c and R2c, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1cR2cN represents an optionally substituted monovalent cyclic hydrocarbon residue,
can be reacted with a compound of general formula (II-C):
R3cCNxe2x80x83xe2x80x83(II-C)
wherein
R3c represents a hydrogen atom or an optionally substituted monovalent hydrocarbon residue,
in an inert solvent or without using a solvent in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give an amidine compound of general formula (III-C) and/or a tautomer thereof: 
wherein
R1c, R2c and R3c are as defined above.
The tautomer as used herein refers to a compound of general formula (III-Cxe2x80x2): 
which may be generated when R2c in general formula (III-C) is a hydrogen atom. Exactly the same can be said for the case where R1c in general formula (III-C) is a hydrogen atom.
Preparation of Amides
A compound of general formula (I-D):
R1dR2dNHxe2x80x83xe2x80x83(I-D)
wherein
R1d and R2d, which are the same or different, each represent a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, or R1dR2dN represents an optionally substituted monovalent cyclic hydrocarbon residue,
can be reacted with an ester compound (excluding highly reactive esters) of general formula (II-D):
R3dCO2R4dxe2x80x83xe2x80x83(II-D)
wherein
R3d represents a hydrogen atom or an optionally substituted monovalent hydrocarbon residue, R4d represents a hydrogen atom, an optionally substituted monovalent hydrocarbon residue or a substituted silyl group, or R3dCO2R4d represents an optionally substituted cyclic hydrocarbon,
in an inert solvent or without using a solvent in the presence of a silylating agent and, if necessary, in the presence of an acid and/or a base to give an amide compound of general formula (III-D): 
wherein
R1d, R2d and R3d are as defined above.
As used herein, highly reactive esters are intended to mean active compounds capable of reacting with amines in the absence of a particular catalyst to yield the corresponding amides. Examples include compounds of general formula (II-D) wherein Rd is pentafluorophenyl or paranitrophenyl. Preparation methods using such ester compounds are not intended to be within the scope of the present invention. The conditions under which these four types of reactions occur will be described below in more detail.
Examples of an inert solvent include hexane, cyclohexane, benzene, toluene, diethyl ether, diisopropyl ether, tert-butyl methyl ether, tetrahydrofuran, dioxane, dichloromethane, 1,2-dichloroethane and chloroform. Preferred for use are dichloromethane, 1,2-dichloroethane, benzene and toluene, or mixtures thereof.
The reaction may be performed at a temperature ranging from xe2x88x9278xc2x0 C. to the boiling point of the reaction mixture, preferably xe2x88x9220xc2x0 C. to 110xc2x0 C.
Examples of the silylating agent available for use include those which are represented by the following formulae: RaRbRcSiX, RaRbRcSiOCORd, RaRbRcSiOSO2Rd, RaRbSi(OSO2Rd)2, (RaRbRcSiO)3-nXnPO, (RaRbRcSi)RfNCORg, (RaRbRcSi)NRiRj and (RaRbRcSiO)(RaRbRcSiN)CRk, wherein Ra, Rb and Rc, which are the same or different, each represent an optionally substituted linear, branched or cyclic C1-C10, alkyl group, an optionally substituted phenyl group or a halogen atom; X represents a halogen atom; Rd represents a hydrogen atom, an optionally substituted linear, branched or cyclic C1-C10 alkyl group, an optionally substituted phenyl group, a halogen atom or RaRbRcSiO; n is 0, 1 or 2; Rf represents RaRbRcSi, a hydrogen atom, an optionally substituted linear, branched or cyclic C1-C10 alkyl group or an optionally substituted phenyl group; Rg represents a hydrogen atom, an optionally substituted linear, branched or cyclic C1-C10 alkyl group, an optionally substituted phenyl group, RaRbRcSiO or (RaRbRcSi)RfN; Ri and Rj, which are the same or different, each represent a hydrogen atom, an optionally substituted linear, branched or cyclic C1-C10 alkyl group, an optionally substituted phenyl group or RaRbRcSi, or NRiRj represents a ring-forming substituent; and Rk represents a hydrogen atom, an optionally substituted linear, branched or cyclic C1-C10 alkyl group, an optionally substituted phenyl group or RaRbRcSiO. These silylating agents may be commercially available or may be prepared in a known manner. Specific examples include Me3SiCl (Me3Si being hereinafter referred to as TMS), Et3SiCl (Et3Si being hereinafter referred to as TES), tBuMe2SiCl (tBuMe2Si being hereinafter referred to as TBS), Me2PhSiCl, TMSO2CCF3, TESO2CCF3, TBSO2CCF3, TMSO3SCl, (TMSO)2SO2, TMSO3SCF3, TESO3SCF3, TBSO3SCF3, (TMSO)3PO, (TMSNH)2CO, (TBSNH)2CO, TMSNMeCHO, (TMS)2NCHO, TMSNMeCOMe, (TMS)2NCOMe, TMSNMeCOCF3, TBSNMeCOCF3, TMSNHCO2TMS, (TMS)2NH, (TMS)3N, 1-TMSimidazole, 1-TBSimidazole, (TMSO)(TMSN)CMe, (TBSO)(TBSN)CMe and (TMSO)(TMSN)CCF3, with TMSCl, (TMSO)2SO2, TMSO3SCF3, TESO3SCF3, TBSO3SCF3 and (TMSO)(TMSN)CMe being preferred. These agents may be used alone or in combination in an amount of 0.1 to 10 equivalents, preferable 1 to 4 equivalents, per amino group present in starting materials.
Further, the reaction may be performed in the presence of an acid and/or a base, if necessary. The acid may be any one of commonly-used Lewis acids and proton acids, or may be any one of inorganic acids (e.g., HCl, H2SO4, H3PO4) and organic acids (e.g., CF3CO2H, CF3SO3H, MeSO3H). The base may be selected from inorganic bases (e.g., K2CO3, Na2CO3, KHCO3, NaHCO3), organic bases (e.g., pyridine, Et3N, Et2iPrN, N,N-dimethylaminopyridine) and organic metals (e.g., MeLi, nBuLi, sBuLi, tBuLi, MeMgCl, MeMgBr, EtMgCl, EtMgBr, iPrMgCl, iPrMgBr, tBuMgCl, tBuMgBr).
As used herein, the optionally substituted monovalent hydrocarbon residue as R1a, R2a, R3a, R1b, R2b, R3b, R4b, R1c, R2c, R3c, R1d, R2d, R3d and R4d refers to a saturated or unsaturated C1-C30 hydrocarbon residue which may be linear, branched or cyclic and may further contain a heterocyclic ring.
Examples include:
alkyl groups such as methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, i-propyl, i-butyl, sec-butyl, tert-butyl, i-pentyl, neopentyl, tert-pentyl, i-hexyl, cyclopropyl, cyclopropylmethyl, cyclobutyl, 2-cyclobutylethyl, cyclopentyl, 1-cyclohexylnonyl, cycloheptyl and cyclooctyl;
alkenyl groups such as vinyl, 1-propenyl, 1-butenyl, 1-pentenyl, 1-hexenyl, allyl, 2-butenyl, 2-pentenyl, 2-hexenyl, 3-butenyl, 3-pentenyl, 3-hexenyl, 1-cyclopropenyl, 2-cyclopropenyl, 1-cyclobutenyl, 2-cyclobutenyl, 1-cyclopentenyl, 2-cyclopentenyl, 3-cyclopentenyl, 1-cyclohexenyl, 2-cyclohexenyl, 1-cyclopropenyl, 1-methylvinyl, 1-methyl-1-cyclopropenyl, 1-methyl-3-cyclopentenyl, 3-methyl-2-pentenyl, 3-methyl-2-cyclohexenyl, 1-ethyl-1-hexenyl and 1-ethyl-2-cyclohexenyl;
alkynyl groups such as ethynyl, 1-propynyl, 1-butynyl, 1-pentynyl, 1-hexynyl, 2-propynyl, 2-butynyl, 2-pentynyl, 2-hexynyl, 3-butynyl, 3-pentynyl, 3-hexynyl, 1-methyl-2-propynyl, 3-methyl-1-butynyl, 1-ethyl-2-propynyl, 2-cyclooctynyl, 3-cyclodecynyl and 1-propyl-2-cyclotridecynyl; and
aromatic groups such as phenyl, naphthyl, anthryl, phenanthryl and pyrenyl.
The heterocyclic ring refers to a saturated or unsaturated cyclic hydrocarbon containing one or more heteroatoms (e.g., nitrogen, oxygen, sulfur) as its ring members. Examples include aziridine, oxirane, thiirane, azetidine, oxetane, pyrrolidine, oxolane, thiolane, pyrrole, furan, thiophene, pyrazolidine, imidazoline, isoxazoline, oxazole, isothiazole, thiazole, pyridine, pyran, pyrimidine, pyrazine, indoline, benzofuran, benzothiophene, benzoxazole, chroman, isoquinoline, quinoxaline, carbazole and acridine.
As used herein, the cyclic hydrocarbon in the optionally substituted monovalent cyclic hydrocarbon residue as R1aR2aN, R1bR2bN, R3bR4bN, R1cR2cN and R1dR2dN refers to a saturated or unsaturated cyclic C1-C20 hydrocarbon which may contain, as its ring members, a heteroatom(s) in addition to the nitrogen atom. Examples include aziridine, azetidine, pyrrolidine, pyrrole, pyrazolidine, imidazoline, oxazolidine, isoxazolidine, isothiazolidine, piperazine, morpholine, indole, dihydroisoquinoline and carbazole.
As used herein, the substituent of the substituted silyl group as R4d refers to a linear, branched or cyclic C1-C10 alkyl group or a C6-C10 aryl group. Examples include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, i-propyl, i-butyl, s-butyl, t-butyl, cyclohexyl and phenyl.
As used herein, the cyclic hydrocarbon as R3dCO2R4d refers to a saturated or unsaturated cyclic C2-C20 hydrocarbon which may contain, as its ring members, a heteroatom(s) in addition to the oxygen atoms. Examples include tetrahydro-2-furanone, tetrahydro-2-pyrone, coumarin, isocoumarin, 2(3H)-benzofuranone and phthalide.
As used herein, examples of the substituent of the optionally substituted monovalent hydrocarbon residue as R1a, R2a, R3a, R1b, R2b, R3b, R4b, R1c, R2c, R3c, R1d, R2d and R3d include a halogen atom, xe2x80x94SiReRfRg, xe2x80x94CONRiRj, xe2x80x94CO2H, xe2x80x94NO2, xe2x80x94N3, xe2x80x94NRmRn, xe2x80x94ORp, xe2x95x90O, xe2x80x94S(O)nRq, xe2x95x90S and xe2x80x94P(O)(ORx)(ORy). Re, Rf and Rg in SiReRfRg each represent a hydrogen atom, a halogen atom, a linear, branched or cyclic C1-C5 alkyl group or an optionally substituted C6-C15 phenyl group. Examples of a group represented by xe2x80x94SiReRfRg include trimethylsilyl, triethylsilyl, t-butyldimethylsilyl, dimethylphenylsilyl and chlorodimethylsilyl.
Ri and Rj in CONRiRj each represent a hydrogen atom or a saturated or unsaturated monovalent C1-C20 hydrocarbon residue which may have a substituent(s) at any position and may be linear, branched or cyclic and may further contain a heteroatom(s) or a heterocyclic ring, or NRiRj represents a saturated or unsaturated cyclic C2-C20 hydrocarbon which may have a substituent(s) at any position. Examples of a group represented by NRiRj include amino, methylamino, benzylamino, ethylamino, dimethylamino, ethylmethylamino, pyrrolidinyl, piperidino, morpholino, acetamide, benzamide, N-methylacetamide, benzamide, t-butoxycarbonylamino, N-methyl-t-butoxycarbonylamino, methylsulfonylamino, ethylsulfonylamino, propylsulfonylamino, isopropylsulfonylamino, butylsulfonylamino, isobutylsulfonylamino, phenylsulfonylamino, p-tolylsulfonylamino and p-chlorophenylsulfonylamino.
Rm and Rn in NRmRn each represent a hydrogen atom or a saturated or unsaturated monovalent C1-C20 hydrocarbon residue which may have a substituent(s) at any position and may be linear, branched or cyclic and may further contain a heteroatom(s) or a heterocyclic ring, or NRmRn represents a saturated or unsaturated cyclic C2-C20 hydrocarbon which may have a substituent(s) at any position. Examples of a group represented by NRmRn include amino, methylamino, benzylamino, ethylamino, dimethylamino, ethylmethylamino, pyrrolidinyl, piperidino, morpholino, acetamide, benzamide, N-methylacetamide, benzamide, t-butoxycarbonylamino, N-methyl-t-butoxycarbonylamino, methylsulfonylamino, ethylsulfonylamino, propylsulfonylamino, isopropylsulfonylamino, butylsulfonylamino, isobutylsulfonylamino, phenylsulfonylamino, p-tolylsulfonylamino and p-chlorophenylsulfonylamino.
Rp represents a hydrogen atom or a saturated or unsaturated monovalent C1-C20 hydrocarbon residue which may have a substituent(s) at any position and may be linear, branched or cyclic and may further contain a heteroatom(s) or a heterocyclic ring. Examples of such a hydrocarbon residue include methyl, isopropyl, t-butyl, benzyl, p-methoxybenzyl, p-methoxyphenyl, cyclopropylmethyl, methoxymethyl, ethoxymethyl, benzyloxymethyl, methylthiomethyl, ethylthiomethyl, methoxycarbonyl, tetrahydropyranyl, tetrahydrofuranyl, trimethylsilyl, triethylsilyl, t-butyldimethylsilyl and dimethylphenylsilyl.
In S(O)nRq, n is 0, 1 or 2 and Rq represents a saturated or unsaturated monovalent C1-C20 hydrocarbon residue which may have a substituent(s) at any position and may be linear, branched or cyclic and may further contain a heteroatom(s) or a heterocyclic ring. Examples of such a hydrocarbon residue include methyl, ethyl, isopropyl, phenyl, p-tolyl, p-chlorophenyl and benzyl.
Rx and Ry in P(O)(ORx)(ORy) each represent a hydrogen atom or a saturated or unsaturated monovalent C1-C20 hydrocarbon residue which may have a substituent(s) at any position and may be linear, branched or cyclic and may further contain a heteroatom(s) or a heterocyclic ring. Examples of such a hydrocarbon residue include methyl, ethyl, isopropyl, t-butyl, phenyl, p-tolyl, p-chlorophenyl, p-methoxyphenyl, benzyl, p-methoxybenzyl, methoxymethyl, tetrahydropyranyl and t-butyldimethylsilyl.